Research Project ? Project Summary A sex ratio of approximately 4:1 in prevalence is nearly universally observed in familial ASD, despite marked heterogeneity in its (primarily autosomal) genetic causes. The mechanisms by which penetrance is reduced in females across diverse autosomal causes of ASD liability?currently referred to as the ?female protective effect? (FPE)5-9 --- remain unknown. Elucidating the mechanisms of FPE will advance our understanding of a host of heterogeneous causes of ASD and holds the potential for illuminating novel and highly potent interventions for a majority of ASD-affected children. The overarching goals of this research project?which is designed to capitalize upon the infrastructure of the IDDRC@WUSTL)--are to predict and elucidate the mechanisms of sex-specific modulation of susceptibility to ASD. The strategy is to take 3 logical (and related) ?next steps? in understanding FPE?which may operate at the level of cell, brain, and/or behavior, and across disparate autosomal causes?to inform new intervention targets relevant to a diversity of familial autistic syndromes. Specific Aim 1 is designed to inform translational advances in risk prediction and genetic counselling for female carriers of inherited ASD susceptibility and their offspring. Using an internet-based registry of over 20,000 ASD-affected families (http://ianproject.org) in which over 2500 unaffected sisters of ASD probands?now of child-bearing age?were historically characterized for quantitative autistic traits, we will test predictions of offspring ASD risk as a function of sex (of infant) and quantitative variation in maternal phenotype. Specific Aim 2 is to determine the extent to which categorical and quantitative variation in expression of the ASD phenotype in ?carrier? adult females relate to previously-published neural (brain MRI) signatures of susceptibility and compensatory function in ASD. Specific Aim 3 is an exploratory aim to develop a preliminary resource for the elucidation of cellular signatures of FPE. We will establish human iPSC-derived neurons from 4 families transmitting separate autosomal ASD-causing variants (each with 2 ASD-affected males and 1 carrier female, 2 cell lines per individual, for a total of 24 cell lines). We will explore whether within-family cellular contrasts are appreciable between affected males and carrier females by examining axon and dendrite morphology as well as the development and function of synapses in the differentiated neurons. If candidate cellular signatures of FPE emerge, we will use these preliminary data to justify subsequent applications (to submit in years 4-5) to pursue a next phase of investigations critically informed by these studies.